Extrusion of powder in capsules has hitherto been hampered because of a number of problems that have been difficult to solve. Among other things, there have been considerable problems with the methods used so far due to a folding of the capsule sheet material. In an article entitled "The Consolidation of Metal Powder by Hot Working within Sheaths" in Powder Metallurgy 1958, Nos. 1, 2, pages 94-103, I. Williams describes different methods of manufacturing products starting from a powder. In one section extrusion is discussed and folding of the capsule sheet is stated to be a serious restriction to the method. To reduce the risk of folding, proposals have been made to isostatically press the capsule, prior to the extrusion, under cold conditions so as to obtain a density of at least 80% of the theoretical density. Such a compression is possible for materials which give a soft powder during granulation, but not for materials which give a hard, spherical powder. A capsule filled with high-speed tool steel powder cannot be compressed so as to obtain sufficient density.
The invention makes it possible to extrude a powder-filled capsule without prepressing the capsule in the cold state and with a good result, even though the density of powder within the capsule is that which is obtainable by vibrating the capsule while filling it. The invention thus makes possible the extrusion of hard, spherical powders that cannot be densified by cold isostatic pressing. The density obtainable through vibration of a spherical powder is normally between 67 and 72% of the theoretical density.